Method for determining the power of an electric motor of a hybrid compressor

ABSTRACT

The invention relates to a method for determining the power (W elec ) of an electric motor ( 20 ) for driving a hybrid compressor ( 10; 10′ ) of an air-conditioning circuit of a motor vehicle having a heat engine during phases in which the drive of the hybrid compressor by the heat engine is interrupted. According to the invention, said method includes steps that consist of: defining a maximum temperature (T max ) measured at a point ( 13 ) of said air-conditioning circuit; setting a reference duration (t ref ) for the phases during which the drive of the hybrid compressor ( 10; 10′ ) by the heat engine is interrupted; determining the power (W elec ) of the electric motor ( 20 ) such that, at the end of said reference duration (t ref ), the temperature measured at said point ( 13 ) of the air-conditioning circuit is at most equal to said maximum temperature (T max ). The invention can be used for air-conditioning motor vehicles having a heat engine and provided with an automatic stopping and restarting system.

The present invention relates to a method for determining the power of an electric motor of a hybrid compressor of an air-conditioning circuit of an engined motor vehicle.

The invention finds a particularly advantageous application in the field of the air-conditioning of engined motor vehicles equipped with an automatic stopping and restarting system, such as the systems able to implement the function known by the term “Stop and Start”.

The “Stop and Start” function consists, under certain conditions, in automatically causing the complete stopping of the engine when the vehicle itself has stopped, and then in automatically restarting the engine following, for example, an action of the driver interpreted as a restart request.

A typical situation for implementing the “Stop and Start” function is that of stopping at a red light. When the vehicle stops at the light, the “Stop” mode of the “Stop and Start” function causes the automatic stopping of the engine, and the vehicle then enters the “Start” mode which allows the engine to restart automatically without it being necessary to use the means for initial starting of the motor, such as a contact key for example. When the light turns green, the “Start” mode automatically restarts the motor, especially by means of an alternator-starter, following the detection by the command system upon the starting of the vehicle of the depression by the driver of the clutch pedal, of the accelerator pedal, or else of any other action that can be interpreted as the driver's desire to restart his vehicle. The benefit of the “Stop and Start” function is understood in terms of energy saving and pollution reduction, particularly in urban surroundings.

Moreover, it is known that an air-conditioning circuit of an engined vehicle comprises a refrigerant fluid compressor which is driven by the shaft of the crankshaft of the engine by way of a belt and a pulley linked mechanically to the rod of the compressor. Stated otherwise, the air-conditioning circuit of the vehicle can only operate if the engine is driving the compressor. Consequently, during the phases of interruption of driving of the compressor by the engine, especially the vehicle stopping phases in the context of the “Stop and Start” function, the compressor is no longer driven by the engine and the air-conditioning ceases to operate. It follows from this that in the course of these stopping phases the setpoint temperature inside the cabin may not be maintained, and this can cause a feeling of discomfort for the passengers of the vehicle.

To ensure maintenance of the temperature in the cabin during the phases of stopping of driving of the compressor by the engine, it is proposed to replace, for example, the usual compressor driven by the engine of the vehicle by a hybrid compressor consisting of two separate compression chambers, constituting, on the one hand, a so-called mechanical compressor whose rod is driven by the engine in the same manner as the usual compressor and, on the other hand, a so-called electric compressor whose rod is driven by an auxiliary electric motor. The rods of two compression chambers are independent.

When the engine is running, outside of the stopping phases determined by the “Stop and Start” function, the refrigerant fluid circulates in the air-conditioning circuit through the mechanical compressor driven by the shaft of the crankshaft of the engine, while the electric compressor is turned off. Conversely, during the phases of stopping of the “Stop and Start” function, the refrigerant fluid is directed toward the electric compressor, which is then driven by the electric motor. Thus, by virtue of the electric compressor, the continuity of operation of the air-conditioning circuit and the maintaining of the comfort temperature in the cabin are carried out when the engine has stopped.

It is necessary, however, to note that during the phases of stopping of the air-conditioning circuit, especially the engine stopping phases imposed by the “Stop and Start” function, the cabin is in general already conditioned in comfort conditions, so that the refrigerative power to be provided by the electric motor so as to maintain these conditions for a duration limited to a few tens of seconds is lower, at least by a factor of 2 to 3, than the power that must be provided by the engine. It is therefore possible to use for the electric compressor a compression chamber of reduced capacity.

In this context of capacity reduction, there is also proposed a hybrid compressor comprising a single variable-capacity compression chamber comprising a compression rod able to be driven either by the engine, under normal operating conditions of the air-conditioning circuit, or by the electric motor during phases of interruption of driving of the compressor by the engine, especially the engine stopping phases determined by the “Stop and Start” function. There is then provision for the capacity of the compression chamber to be able to vary in a span of variation exhibiting a higher interval of capacities, in which the compression chamber is driven by the engine, and a lower interval of capacities, in which the compression chamber is driven by the electric motor.

The operation of an air-conditioning circuit of a motor vehicle customarily comprises a first so-called temperature drop phase (also known by the term “cool-down”) which occurs upon the starting of the vehicle after the latter has remained stopped for a long enough duration for the temperature inside the cabin to exceed the comfort temperature sought by the occupants of the vehicle, around 18° C. for example. During this temperature drop phase, the thermal power to be provided in order to reach the comfort temperature is relatively high, of the order of 6 kW when the vehicle has been exposed to a temperature of 25 to 45° C. under sunshine of 1000 W·m² and a relative humidity of 50 to 60%.

At the level of the air-conditioning circuit proper, the temperature drop conditions are defined in such a way that, when the comfort temperature is reached, the evaporator of the circuit is at a setpoint temperature of between 4 and 8° C., which temperature best guarantees the elimination of a large part of the moisture contained in the air blown into the cabin, as well as the limitation of the development of the bacteria responsible for the bad odors that may be experienced in the cabin. However, a maximum temperature on the evaporator of between 12 and 16° C. remains acceptable, since it is only beyond this temperature, the so-called air-conditioning discomfort threshold, that the occupants of the vehicle notice the unpleasantness related to humidity and to bacteria.

The first phase of temperature drop is followed by a second phase of maintaining the comfort temperature, characterized by a thermal power to be provided which is substantially lower, of the order of 3 kW, than that of about 6 kW which was necessary in order to bring the cabin to the comfort temperature.

In the normal compressor drive regime, it is the engine which is responsible for maintaining the temperature of the cabin at the comfort temperature.

During the phases of interruption of the driving of the compressor by the engine, and in accordance with the principle of the hybrid compressor, it is the electric motor which picks up the burden of maintaining the comfort conditions in the cabin. In this case, the electrical power to be provided to the electric motor for a maintaining thermal power of 3 kW is about 1500 W.

If, for hybrid vehicles of the “Mild Hybrid” and “Full Hybrid” sectors equipped with high-voltage (60 to 150 V for “Mild Hybrid” vehicles, 500 to 600 V for “Full Hybrid” vehicles) direct current networks, providing a power of 1500 W to the electric motor of the hybrid compressor does not pose any problem, matters are otherwise for vehicles of the “Micro Hybrid” sector, equipped with an engine and an alternator-starter for the implementation of the “Stop and Start” function, these vehicles only having a 12 V low-voltage network. The intensity of current demanded is then too high and the required maintaining power may not be provided to the electric motor by the onboard network. Under these conditions, it is not possible to guarantee the same comfort level during the phases of interruption of driving of the compressor by the engine as during the phases of maintaining the comfort temperature by the engine.

Hence, an aim of the invention is to propose a method for determining the power of an electric motor intended to drive a hybrid compressor of an air-conditioning circuit of an engined motor vehicle during phases of interruption of driving of the hybrid compressor by the engine, which would nonetheless make it possible to be able to maintain in the cabin of the vehicle a sufficient comfort level during the phases of interruption of driving by the engine, even in the context of vehicles of the “Micro Hybrid” sector having only a low-voltage network to supply the electric motor of the hybrid compressor.

This aim is achieved, in accordance with the invention, because said method comprises steps consisting in:

-   -   defining a maximum temperature measured at a point of said         air-conditioning circuit,     -   fixing a reference duration for the phases of interruption of         driving of the hybrid compressor by the engine,     -   determining the power of the electric motor in such a way that         on completion of said reference duration, the temperature         measured at said point of the air-conditioning circuit is at         most equal to said maximum temperature.

In practice, the invention provides that said maximum temperature is measured at the evaporator of the air-conditioning circuit, and that said maximum temperature is the air-conditioning discomfort threshold.

The reference duration is defined as a duration compatible with the durations generally observed for the phases of interruption of driving by the engine. This reference duration may be for example 35 s.

It is thus understood that the electric motor power determined in accordance with the method according to the invention may be rendered substantially lower than that which would be necessary in order to maintain the temperature of the evaporator at the setpoint temperature of 4 to 8° C., which, as has been seen, was of the order of 1500 W. Indeed, if, within the framework of the invention, it is accepted for example that the temperature measured on the evaporator may reach the air-conditioning discomfort threshold of 12 to 16° C. on completion of the reference duration of 35 s, it then suffices to use an electric motor of lower power, of 500 to 800 W, that can be supplied directly by the 12 V low-voltage network of the vehicle.

According to the invention, the ratio of the power of the electric motor to the power necessary for the electric motor to maintain the temperature at said point of the air-conditioning circuit equal to said temperature at the commencement of a phase of interruption of driving by the engine lies between 0.2 and 0.8. More specially, said ratio lies between 0.3 and 0.55.

In a first application of the method according to the invention, said interruption of driving is a stopping of the engine. In particular, said stopping of the engine is an automatic stopping determined by a function for automatic stopping and restarting of the engine of the vehicle (“Stop and Start”).

In a second application of the method according to the invention, said interruption of driving is a decoupling of the engine from the hybrid compressor determined by a vehicle acceleration request.

The description which follows with regard to the appended drawings, which are given by way of nonlimiting examples, will elucidate the invention and the manner in which it may be embodied.

FIG. 1 is a diagram of an air-conditioning circuit comprising a hybrid compressor with two compression chambers.

FIG. 2 is a diagram of an air-conditioning circuit comprising a hybrid compressor with a single compression chamber.

FIG. 3 is a chart showing the evolution of the temperature of the evaporator of an air-conditioning circuit with hybrid compressor for various operating phases of the compressor.

In FIG. 1 is represented a conventional air-conditioning circuit of an engined motor vehicle, comprising a compressor 10 of a refrigerant fluid which may be an organic, inorganic or eutectic fluid. It is possible to cite as nonlimiting examples supercritical carbon dioxide CO₂, the refrigerants known by the references R134A, 1234yf or else GAR (“Global Alternative Refrigerant”). Downstream of the compressor 10, the pressurized refrigerant fluid passes through a heat exchanger 11 called a “gas cooler” for carbon dioxide or a “condenser” for R134A since, in this case, the refrigerant initially in the gas phase exits the condenser in liquid form.

In the example of FIG. 1, the exchanger 11 may be a water-type exchanger, or an air-type exchanger cooled directly by the outside air.

The refrigerant fluid is thereafter conducted toward a relief valve 12 so that it is cooled before entering the evaporator 13 where heat exchange then occurs between the cooled refrigerant and air blown toward the cabin of the vehicle.

The refrigerant fluid, reheated on exit from the evaporator 13, is then returned to the compressor 10 to perform a new thermal cycle.

As may be seen in FIG. 1, the compressor 10 of FIG. 1 is a hybrid compressor of the type with two separate compression chambers, namely, on the one hand, a first chamber 101 comprising a first compression rod 111 able to be driven by the shaft of the crankshaft of the engine (not represented) of the vehicle via a belt and a pulley 30 linked mechanically to the rod 111 by way of a clutch 31, and, on the other hand, a second chamber 102 comprising a second compression rod 112, independent of the first rod 111, able to be driven by an electric motor 20.

During nominal operation of the air-conditioning circuit, the rod 111 of the first compression chamber 101 is driven by the engine, the pulley 30 being coupled to the rod 111 by the clutch 31. The refrigerant fluid then circulates through the first chamber 101 whose capacity, of the order of 100 cm³, is chosen so as to allow the hybrid compressor 10 to ensure an optimal comfort level inside the cabin of the vehicle, whatever the outside temperature, the sunshine and the degree of relative humidity.

However, it can happen, in certain circumstances, that the air-conditioning compressor 10 is no longer driven by the engine of the vehicle and that, consequently, the air-conditioning circuit ceases to operate and can no longer guarantee maintenance of the comfort temperature inside the cabin. Such is the case especially during the engine stopping phases determined by a system for automatic stopping and restarting of the engine able to implement the “Stop and Start” function of the vehicles equipped with this function.

In order to ensure continuity of air-conditioning in the cabin, the circulation of refrigerant fluid is switched from the first chamber 101 to the second chamber 102 by a valves device internal to the hybrid compressor 10, and then the electric motor 20 is started so as to drive the second compression rod 112 and maintain the air-conditioning circuit in operation during these stopping phases.

When the electric motor 20 takes over from the then stopped engine, the cabin of the vehicle is in principle already at the comfort temperature, so that, having regard to the fact that the duration of the stopping phases is generally limited to a few tens of seconds, the refrigerative power to be provided by the electric motor 20 is relatively low.

Consequently, the capacity of the second compression chamber 102 may be limited, with respect to the capacity of the first chamber 101, to values of about 20 cm³ for example.

In FIG. 2 is represented another type of hybrid compressor 10′ comprising a variable-capacity compression chamber 100 whose rod 110 may be driven, either by the electric, motor 20, or by the shaft of the crankshaft of the engine (not represented) of the vehicle via a belt and the pulley 30 able to be linked mechanically to the rod 110 by way of the clutch 31.

It is necessary to emphasize here that this architecture of hybrid air-conditioning compressor is distinguished from the compressor of FIG. 1 by the fact that it implements only a single compression chamber and a single rod that can equally well be driven by the engine or by the electric motor, instead of two separate compression chambers of independent rods.

During nominal operation, the rod 110 of the compression chamber 100 is driven by the engine, the pulley 30 being coupled to the rod 110 by the clutch 31. The capacity of the compression chamber is then chosen in a higher interval of values close to the maximum capacity, for example lying between 90 and 110 cm³. Under these conditions, the hybrid compressor 10′ is capable of ensuring an optimal comfort level inside the cabin of the vehicle, whatever the outside temperature, the sunshine and the degree of relative humidity.

Just as for the hybrid compressor 10 with two chambers of FIG. 1, during the engine stopping phases determined by a function for automatic stopping and restarting of the “Stop and Start” type, the air-conditioning compressor 10′ is no longer driven by the engine of the vehicle. The air-conditioning circuit then ceases to operate and no longer guarantees maintenance of the comfort temperature inside the cabin.

Under these conditions, the electric motor 20 is set into operation so as to drive in turn the compression rod 110 and ensure continuity of air-conditioning. Stated otherwise, it may be considered that the electric motor 20 then substitutes itself for the engine in its function of driving the compression chamber 100. Of course, the engine is, preferably, disengaged from the compression rod 110.

It was already mentioned above that the refrigerative power to be provided by the electric motor 20 in operation is relatively low and that, consequently, the capacity of the compression chamber 100 may be reduced, with respect to the nominal operating conditions, to values lying in a lower interval of capacities of about the minimum capacity, for example between 20 and 40 cm³.

Of course, the higher and lower intervals of capacities may be simply reduced to the maximum and minimum capacities alone.

The compression chamber 110 then switches in a binary manner between these two capacities depending on whether the motive drive for the rod of the chamber is the engine or the electric motor.

In FIG. 3 are represented three typical phases of the evolution of the temperature measured on the evaporator 13 of an air-conditioning circuit comprising a hybrid compressor, be it the compressor 10 with two chambers of FIG. 1 or the compressor 10′ with single chamber of FIG. 2.

The first phase is the temperature drop phase (or “cool-down”) which takes place generally upon the starting of the vehicle when the latter has remained stopped long enough for the temperature of the evaporator 13 inside the cabin to reach relatively high values T_(ext), of 25 to 45° C., greater than the comfort temperature of generally around 18° C.

This temperature drop phase is carried out by means of the engine of the vehicle by driving the first chamber 101 of the hybrid compressor 10 of FIG. 1 or the single chamber 100 of the hybrid compressor 10′ of FIG. 2, the capacity of the chamber 100 being chosen in the higher interval of capacities.

The parametrization of the air-conditioning circuit is defined so that the comfort temperature in the cabin can be reached for values of the setpoint temperature T_(cons) of the evaporator 13 lying between 4 and 8° C., so as to eliminate the moisture of the air introduced into the cabin and avoid the bad odors developed by certain bacteria. The maximum temperature T_(max) acceptable on the evaporator 13 is taken equal to the air-conditioning discomfort threshold lying between about 12 and 16° C., beyond which the odors produced by bacteria become unpleasant to the occupants of the vehicle.

As was seen above, the temperature drop phase requires the production by the engine of a thermal power of the order of 6 kW.

When the temperature of the evaporator 13 has reached the desired setpoint value T_(cons), the air-conditioning circuit enters a second phase of so-called maintaining the comfort temperature.

Under normal operating conditions this maintaining phase is ensured by the engine. Of course, the thermal power to be provided is lower than during the temperature drop phase, about 3 kW instead of 6 kW.

On the other hand, during the phases of stopping of the driving of the compressor by the engine, especially the phases of stopping determined by the “Stop and Start” function or of decoupling of the motor from the compressor following a vehicle acceleration request, it is the electric motor 20 which must ensure maintenance of comfort in the vehicle. To provide the required thermal power of 3 kW, an electrical power W₀ of about 1500 W is necessary.

Now, it was explained above that electrical power such as this is difficult to provide for “Micro Hybrid” vehicles with engine and alternator-starter which have only a 12 V low-voltage electrical network.

Hence, the invention proposes a method for determining a power of the electric motor 20 which would be compatible with a 12 V onboard network, but which would nonetheless guarantee the occupants of the vehicle a sufficient feeling of comfort for the duration, generally short, of the stopping phases envisaged here.

As indicated by FIG. 3, after having defined the maximum temperature T_(max) not to be exceeded at a point of the air-conditioning circuit, here a temperature of to 16° C. measured on the evaporator 13 of the circuit, a reference duration t_(ref) is fixed for the phases of interruption of driving by the engine. This duration t_(ref) may be determined by the average of the durations observed for the interruption phases, for example 35 s.

Next, the electrical power W_(elec) of the electric motor 20 is defined in such a way that on completion of the reference duration t_(ref) the temperature measured on the evaporator 13 is at most equal to the maximum temperature T_(max). In the example shown in FIG. 3, the temperature of the evaporator is taken equal to the temperature T_(max).

Under these conditions, the electrical power W_(elec) may be reduced to values of for example between 500 and 800 W, compatible with the 12 V voltage of the low-voltage onboard network of the vehicles of the target sector, while avoiding the emanation of bad odors which could occur at higher temperatures.

In a general way, for a value of 1500 W of the electrical power W₀ to be provided in order to maintain the evaporator 13 at the setpoint temperature T_(cons), the power determined by the method of the invention is in a ratio of 0.2 to 0.8, and, preferably, between 0.3 and 0.55.

It is also necessary to point out that the method in accordance with the invention for determining the power of the electric motor of a hybrid compressor also applies to “Mild Hybrid” and “Full Hybrid” vehicles since even for these vehicles a decrease in the electrical energy to be provided by the onboard network constitutes an advantage which may not be ignored. 

1. A method for determining the power of an electric motor intended to drive a hybrid compressor of an air-conditioning circuit of an engine motor vehicle during phases of interruption of driving of the hybrid compressor by the engine, the method comprising: defining a maximum temperature measured at a point of said air-conditioning circuit; fixing a reference duration for the phases of interruption of driving of the hybrid compressor by the engine; and determining the power of the electric motor so that on completion of said reference duration, the temperature measured at said point of the air-conditioning circuit is at most equal to said maximum temperature.
 2. The method as claimed in claim 1, wherein said maximum temperature is measured at the evaporator of the air-conditioning circuit.
 3. The method as claimed in claim 2, wherein said maximum temperature is the air-conditioning discomfort threshold.
 4. The method as claimed in claim 1, wherein the ratio of a power of the electric motor to a power necessary for the electric motor to maintain the temperature at said point of the air-conditioning circuit equal to said temperature at commencement of a phase of interruption of driving by the engine lies between 0.2 and 0.8.
 5. The method as claimed in claim 4, wherein said ratio lies between 0.3 and 0.55.
 6. The method as claimed in claim 1, said hybrid compressor comprises a first compression chamber able to be driven by said engine and a second compression chamber able to be driven by said electric motor during the phases of interruption of driving of the hybrid compressor by the engine.
 7. The method as claimed claim 1, wherein said hybrid compressor comprises a variable-capacity compression chamber, able to be driven by the engine in a higher interval of capacities and by the electric motor in a lower interval of capacities during the phases of interruption of driving of the hybrid compressor by the engine.
 8. The method as claimed in claim 1, wherein said interruption of driving is a stopping of the engine.
 9. The method as claimed in claim 8, wherein said stopping of the engine is an automatic stopping determined by a function for automatic stopping and restarting of the engine of the vehicle (“Stop and Start”).
 10. The method as claimed in claim 1, wherein said interruption of driving is a decoupling of the engine from the hybrid compressor determined by a vehicle acceleration request. 